The fast-moving national reckoning over sexual harassment in the workplace toppled another television news star on Wednesday when NBC fired Matt Lauer, the co-host of its most profitable franchise, “Today,” after an allegation of inappropriate sexual behavior with a subordinate.

The downfall of Mr. Lauer, a presence in American living rooms for more than 20 years, adds to a head-spinning string of prominent firings over sexual harassment and abuse allegations. NBC News said it had decided to dismiss its star morning anchor after a woman met with network executives on Monday to describe her interactions with him.

“While it is the first complaint about his behavior in the over 20 years he’s been at NBC News, we were also presented with reason to believe this may not have been an isolated incident,” Andrew Lack, the NBC News chairman, wrote in a memo to staff.

On Wednesday, NBC received at least two more complaints related to Mr. Lauer, according to a person briefed on the network’s handling of the matter. One complaint came from a former employee who said Mr. Lauer had summoned her to his office in 2001, locked the door and sexually assaulted her. She provided her account to The New York Times but declined to let her name be used.

She told The Times that she passed out and had to be taken to a nurse. She said that she felt helpless because she didn’t want to lose her job, and that she didn’t report the encounter at the time because she felt ashamed.

On Thursday, Mr. Lauer issued an apology, expressing “sorrow and regret for the pain I have caused.”

News of Mr. Lauer’s sudden downfall shook the television world, where he had established himself as one of the most powerful men in his industry. Even President Trump — who himself has denied multiple allegations of sexual misconduct — weighed in, seizing on Mr. Lauer’s firing to denounce NBC News’s coverage and call for other senior figures at NBC News to be ousted.

Mr. Lauer, 59, joins an ignominious group of media figures felled by the recent spate of harassment claims, including the studio mogul Harvey Weinstein, the comedian Louis C.K., the CBS host Charlie Rose and the political journalist Mark Halperin. Journalists at several news outlets had recently conducted interviews with former and current NBC employees about Mr. Lauer’s behavior, alerting the network to potential articles about him. But it was the formal complaint on Monday that prompted NBC to take action.

In an editorial meeting on Wednesday, Mr. Lack said that Mr. Lauer’s involvement with the woman who made the complaint began while they were in Sochi, Russia, to cover the Winter Olympics in 2014, and that their involvement continued after they returned to New York, according to two people briefed on the meeting.

Other “Today” hosts learned of Mr. Lauer’s termination around 4 a.m. on Wednesday; staff members were told just minutes before the show went on the air at 7 a.m. Savannah Guthrie, Mr. Lauer’s co-anchor, was visibly shaken when she delivered the news to viewers, describing Mr. Lauer as “a dear, dear friend” and adding that she was “heartbroken for the brave colleague who came forward to tell her story.”

Soon after announcing the dismissal, Ms. Guthrie gripped the hand of Hoda Kotb, who was rushed in as an emergency substitute host. The network did not name a replacement for Mr. Lauer.

Ari Wilkenfeld, a civil rights lawyer with the firm Wilkenfeld, Herendeen & Atkinson in Washington, said on Wednesday that he represented the woman who had made the initial complaint to NBC, but declined to identify her. In a statement provided to The Times, he praised the courage of his client and said:

“My client and I met with representatives from NBC’s human resources and legal departments at 6 p.m. on Monday for an interview that lasted several hours. Our impression at this point is that NBC acted quickly, as all companies should, when confronted with credible allegations of sexual misconduct in the workplace.”

The woman met with reporters from The Times on Monday, but said she was not ready to discuss it publicly.

Besides his “Today” perch, Mr. Lauer was a genial co-host of events like the Macy’s Thanksgiving Parade and the Winter and Summer Olympics, and he conducted countless interviews with celebrities. He also contributed to NBC News’s political coverage, although he was widely panned after a debate last year in which he appeared to go easy on Mr. Trump while asking aggressive questions of Hillary Clinton.

The “Today” show caters to — and relies on — an overwhelmingly female audience, and Mr. Lauer is part of a cast that presents itself as a tight-knit family. Behind the scenes, however, the on-set environment could sometimes resemble a boys’ club, particularly in the years before Comcast completed its acquisition of NBCUniversal in 2013, according to interviews with more than half a dozen former staff members.

Jokes about women’s appearances were routine, the former employees said. One former producer recalled a director saying he “wanted some milk” in reference to one woman’s chest and making inappropriate comments about women over an audio feed with multiple people listening. Two former employees recalled colleagues playing a crude game in which they chose which female guests or staff members they would prefer to marry, kill or have sex with.

The former employees spoke anonymously because they feared their career prospects in the industry could be harmed.

Other current and former staff members, however, described a more professional work culture, and said they did not witness harassment. An NBC spokeswoman declined on Wednesday to comment on the “boys’ club” characterization, but pointed out that 13 of 19 senior-level female producers at “Today” had been promoted since 2015.

The woman who described the encounter in 2001 with Mr. Lauer in his office told The Times that the anchor had made inappropriate comments to her shortly after she started as a “Today” producer in the late 1990s.

While traveling with Mr. Lauer for a story, she said, he asked her inappropriate questions over dinner, like whether she had ever cheated on her husband. On the way to the airport, she said, Mr. Lauer sat uncomfortably close to her in the car; she recalled that when she moved away, he said, “You’re no fun.”

In 2001, the woman said, Mr. Lauer, who is married, asked her to his office to discuss a story during a workday. When she sat down, she said, he locked the door, which he could do by pressing a button while sitting at his desk. (People who worked at NBC said the button was a regular security measure installed for high-profile employees.)

The woman said Mr. Lauer asked her to unbutton her blouse, which she did. She said the anchor then stepped out from behind his desk, pulled down her pants, bent her over a chair and had intercourse with her. At some point, she said, she passed out with her pants pulled halfway down. She woke up on the floor of his office, and Mr. Lauer had his assistant take her to a nurse.

The woman told The Times that Mr. Lauer never made an advance toward her again and never mentioned what occurred in his office. She said she did not report the episode to NBC at the time because she believed she should have done more to stop Mr. Lauer. She left the network about a year later.

On Wednesday, the episode in Mr. Lauer’s office was reported to NBC News after the woman told her then-supervisor, who still works at the network. The woman said an NBC human resources representative had since contacted her.

The woman, who was in her early 40s at the time, told her then-husband about the encounter, which The Times confirmed with him in a phone call. The couple was separated at the time, and later divorced. She also described it to a friend five years ago, which the friend confirmed to The Times.

NBC News has suffered other black eyes, as well. Last year, the network reviewed 2005 footage from the NBC-owned show “Access Hollywood” that revealed Mr. Trump bragging about grabbing women’s genitalia. But the footage was released first by a competitor, The Washington Post, embarrassing the NBC news division.

In recent weeks, NBC News was criticized for passing on an exposé of Mr. Weinstein by an MSNBC contributor, Ronan Farrow. Mr. Farrow’s reporting later appeared in The New Yorker, and helped set off the current wave of revelations about abuses by powerful men in media and entertainment.

A version of this article appears in print on , on Page A1 of the New York edition with the headline: Longtime Face of NBC’s ‘Today’ Is Fired as Complaints Multiply. Order Reprints | Today’s Paper | Subscribe